Light emitting diodes (LEDs), including organic light-emitting diodes (OLEDs) such as polymer OLEDs (PLEDs), small-molecule OLEDs (SMOLEDs) and light emitting electrochemical cells (LEEC), are proposed for several different lighting applications, such as for providing ambient light and as light sources in flat panel displays and signage.
One important category is visual signal application, where the light goes more or less directly from the LED to the human eye, to convey a message or meaning. This typically holds for use in displays and signage, such as electronic billboards, dynamic decorative displays, and thin, lighweight message displays at e.g. airports and railway stations, and as destination displays for e.g. trains, buses, trams, and ferries.
In an OLED, electrons and holes are injected into a layer of electroluminescent semiconducting material where they combine to generate excitons that undergo radiative decay. Holes are injected from the anode into the highest occupied molecular orbital (HOMO) of the electroluminescent material; electrons are injected from the cathode into the lowest unoccupied molecular orbital (LUMO) of the electroluminescent material
OLED devices typically comprise a substrate supporting an anode layer, a cathode layer, and a light emitting layer comprising at least one organic or polymeric electroluminescent compound. The light emitting layer is normally sandwiched between the anode and the cathode. The cathode serves to inject negative charge carriers (electrons) and the anode serves to inject positive charge carriers (holes) into the organic layer.
For displays the device normally is patterned into a plurality of independently addressable domains (hereinafter referred to as pixels). Other layers may be present to enhance the OLED performance. For example, insertion of hole and/or electron injection and transport layer(s) is known to result in improved performance of several types of organic OLEDs.
Thus, a typical OLED comprises two organic layers sandwiched between two conductive electrodes. Counting from the anode, the first of the organic layers is responsible for hole transport and the second layer is responsible for the light generation. Electrons injected by the cathode and holes injected from the anode recombine in the light emitting layer, resulting in an exciton that decays radiatively in producing a photon. The color of the emitted light may be tuned by varying the band-gap of the emissive material used.
OLEDs find particular usage in flat panel and/or flexible displays. An advantage of OLED-based displays, as compared to Liquid Crystal Displays (LCDs) is that they avoid the need for backlight, which makes LCDs high energy consumptive.
In order to address the individual domains several schemes are possible: Passive matrix displays comprise a cross bar array of electrodes, via which each LED-based pixel can be addressed by applying a sufficiently high voltage to the pixels selected, and a relatively low voltage to the unselected pixels. This technology can be applied only in case of a limited number of pixels, and is therewith limited to displays of relatively small size and/or low resolution.
In active matrix displays, the diodes are addressed by providing each pixel with a field-effect transistor, which works as a switch to turn the pixel on or off, dependent on the gate voltage. Although this technology does not lead to the size limitations of a passive matrix display, it is a relatively expensive solution, and increasingly so with the size of the display.
Particularly for signage applications (large outdoor displays serving purposes of information and/or advertisements) the transistor technology is commercially unattractive.
Further, in respect of both of the foregoing types of displays, it would be desired to reduce or avoid the constant electrical energy to not only switch the pixels, but also to keep them in the desired state.